Field of the Invention
The present invention relates to an image sensor, an image sensing device, a camera, and a method of driving the image sensing device.
Description of the Related Art
An image sensor such as a CMOS sensor or CCD sensor operates when a power supply circuit supplies a power supply voltage to the power supply terminal of the sensor. The power supply voltage output from a power supply contains random noise. This random noise propagates to pixels in the image sensor or a readout unit, which reads out signals from pixels, via a power supply line. For this reason, the random noise contained in the power supply voltage supplied from the power supply circuit can decrease the S/N ratio of an image signal output from the image sensor.
Some image sensors are designed to output an image signal corresponding to the difference between a signal corresponding to a reset level from a pixel and a signal corresponding to an optical signal level from the pixel, that is, to perform CDS (Correlated Double Sampling) (see Japanese Patent Laid-Open No. 2003-51989). Although such an image sensor can obtain an image signal having a high S/N ratio, there is a time difference between the timing of sampling a signal corresponding to a reset level and the timing of sampling a signal corresponding to an optical signal level. For this reason, even an image sensor designed to perform CDS has its own limit when removing the influence of random noise from the power supply circuit.